1. Field Of The Invention
This invention relates to the molding of cellular plastic products such as polyurethane foam products and more specifically is directed to a novel and useful process technique applicable to such molded high resilience polyurethane foam which allows the production of more readily crushable foam.
2. Prior Art
In the normal manufacture of molded high resilience polyurethane foam, the chemical reactants required to make the foam part are intimately mixed by mechanical means in accurately metered quantities per a designed stoichiometry and immediately poured into a mold. The mold is then closed and the reactant charge is allowed to expand to fill the intricacies of the mold by displacement of air which is allowed to exit the mold through special vents or, in some cases, a less than air tight mold parting line. The expansion results from gas generation either as a product of reaction of formulation water with the isocyanate to yield CO.sub.2 and/or by volatilization of an auxiliary blowing agent such as a fluorocarbon. Concurrently with this expansion or blowing mechanism, urethane polymers are being formed by the reaction of the polyols and the isocyanate. Normal state of the art technology allows for catalytically balancing these concurrent reactions so as to achieve maximum expansion more or less coincident with gelation or loss of mobility of the plastic mass. The polyurethane then continues to cure until such polymer strength is achieved as to allow manual demolding of a useful foamed part having the shape and dimensions of the mold.
In formal state of the art practice, as the expanding mass in the mold reaches the air vents, some small amount of material will extrude through; however, the on-going polymerization reaction quickly advances to the stage where further fluid flow is effectively precluded. At this state the mold becomes effectively sealed. Further expansion is thereby eliminated although the blowing mechanism may continue to be operative causing a pressure build in the cells which make up the cellular product.
After allowing adequate in-mold curing time, the part that is produced after this molding procedure is considered to be "closed" cell, meaning that a sufficient number of the cells making up the foamed part are closed to the degree that, if the part is allowed to cool to ambient temperature, the part will shrink as internal cell pressures drop below atmospheric. To avoid this post mold shrinkage, the normal procedure is to crush the just demolded and still warm part through some sort of crushing operation. This crushing step serves to burst open the cell windows of the cells allowing the air in the cells to equilibrate to outside pressure and thus avoid shrinking. Typical techniques for crushing include passing the part through rolls that cause one or more deflections of the part of the order of 90% of original thickness. Other techniques sometimes employed involve crushing the foam with a rubber tire as it comes over a belt, vacuum crushing, and air injection. Each of these techniques is currently employed in the industry and have specific utility in bursting cell walls/windows adequately to eliminate shrinkage if the foam is not too "tight", i.e., hard to crush. Foam which is too tight may split in the crusher or may not crush out adequately to preclude shrinkage.
Tight foam can result from several causes both chemical and physical and in many cases those elements of the formulation that cause tight foam are desirable for other reasons, and generally a workable or crushable tightness dictates a compromise in other areas. For example, tight foams are more stable and less likely to exhibit shear induced collapse.
All the methods of foam crushing described above function by means of creation of a pressure differential across cell windows adequate to cause window rupture thereby leaving an open cell. An earlier application, U.S. Ser. No. 549,516 filed Nov. 7, 1983, now U.S. Pat. No. 4,579,700 issued Apr. 1, 1986 describes a unique processing technique wherein a cell rupturing pressure differential is created by means of exposing the molded part to atmospheric pressure. This is done at such a time in the cure cycle that insufficient polymer strength exists in the expanded urethane to contain the internal cell pressures which are greater than atmospheric. The polymer strength is sufficient at this point, however, to prevent the atmospheric pressure from collapsing the foam cells.
This technique, termed "TPR" for Timed Pressure Release, is accomplished while the molded part is still within the mold by the simple expedient of unsealing the mold at the critical time by either unlatching the lid or by opening a special port. When properly practiced, this technique can yield a molded high resilience polyurethane foam part that does not require post mold crushing and will not shrink upon cooling.
Because the mold must be unsealed at a precise point when the foam will not collapse, i.e., because of sufficient polymer strength at this point to withstand collapse, yet there is a sufficient pressure differential across the cell windows to rupture these windows, the timing of this pressure release is critical. If pressure release occurs too early, then foam collapse will occur; if too late, then sufficient polymeric strength will have developed so that the cell windows can withstand the pressure differential resulting in no window rupture and a foam that shrinks or requires crushing after demold.
Studies have shown that essentially all high resilience polyurethane molded formulations have a processing time "window" wherein this technique can effectively eliminate crushing. The TPR window can vary in duration of time as a function of formulation variables that effect system reactivity. Rapid demold systems (where demolding occurs less than 31/2 minutes after the pressure release products are mixed), such as those used in the production of high resilience polyurethane foams, usually have TPR windows so short in duration (as short as 5 seconds has been observed) that application of the TPR technique has limited utility in a production environment.
Thus, it would be very advantageous and a technical advancement over the prior art to expose a curing product to atmospheric or reduced pressure at a point in time somewhere past the TPR window where it is not necessary to worry about the short duration of time of said window, but before the cell walls have developed sufficient strength where exposure to atmospheric pressure would not result in a reduction of the crushing force necessary to prevent said products from substantially shrinking or changing dimensionally.
As mentioned above, a window of short time duration, such as those characteristic in rapid demold systems, is of limited utility in a production environment. This is because foam making is not an exact science; small variations in the exact second of delivery of reactants by the machines and in the exact isocyanate levels used in the foam productions create variations in exactly when the balance between the polymer strength to withstand collapse and total rupture of nearly all cell windows occurs. These variations can be as great as 10-20 seconds. Because the TPR window is of such short duration in rapid demold systems, there is no guarantee that the mold will be unsealed at the precise moment required. With the technique of the present invention, it is not critical that the mold be unsealed at that precise point because it is not necessary that all the cell windows be opened; the present invention is aimed at reducing the force to crush, not at eliminating the crushing step altogether.
Other than the patent application discussed above, no prior art is currently known which utilizes the concept of pressure release by venting to a lower pressure environment during a critical period of polymer formation such that the higher pressure in the cells is sufficient to rupture the cell window yet not strong enough to collapse the growing polymer.
There is one patent, British Pat. No. 1,402,718, which discloses reopening vent holes during the production of foam in a cushion so that excess gas may escape and make the resulting cushion softer. This patent, however, uses very few vents which are additionally quite small in diameter. Moreover, the vents are unplugged one by one resulting in a very small quantity of cells ever being exposed to atmospheric pressure at one time. Thus, the number of cells opened is not appreciable and, in particular, is not greater than 25%.
As noted above, no method is known for using the differences between atmospheric pressure and the higher cell pressures to open cell windows prior to demold, and thereby reducing the force to crush in a post mold crushing step.
3. Objectives
It is thus an object of this invention to teach the use of a process using atmospheric pressure differential at a point past the TPR window, i.e., as is necessary in a rapid demold system with TPR windows of short duration, but before the cell walls have developed sufficient strength so that exposure to the lower atmospheric pressure does not result in a reduction of the crushing force necessary to prevent polyurethane cellular products from substantially shrinking or changing dimensionally.
It is a further object of this invention to provide molded high resilience foams that require less force to crush to preclude substantial shrinking or dimensional change.
It is still a further object of this invention to provide a mechanism to make crushing possible with very "tight" mold foams as are characteristic of highly reactive and/or rapid demold systems.
It is an even further object of this invention to permit greater formulation latitudes by decreasing the constraints normally imposed by foam tightness considerations.
Another object of this invention is to teach the use of a vacuum in this process to increase the pressure differential and thus broaden the latitude of the technique of reducing the force to crush.
Other objects and advantages of the invention will become apparent as the description thereof proceeds.